Hydroxynaphthalenecarboxylic acids are industrially produced by Kolbe-Schmitt process using naphthols converted to the alkali metal salts. The resultant products are obtained as a mixture of alkali metal salts of unreacted materiel and various hydroxynaphthalenecarboxylic acids having carboxyl groups at different positions. For example, 2-hydroxynaphthalene-3-carboxylic acid (hereinafter referred to as BON3) is obtained as a major component in the reaction products, when the sodium salt of 2-naphthol is used in the preparation of hydroxynaphthalenecarboxylic acids from 2-naphthol by Kolbe-Schmitt process. In contrast, 2-hydroxynaphthalene-6-carboxylic acid (hereinafter referred to as BON6) is obtained as a major component in the reaction products when the potassium salt of 2-naphthol is used in such preparation, although the potassium salt yields BON3 as a major component in the reaction products when reacted under high pressure. Other products such as 2-hydroxynaphthalene-3,6-dicarboxylic acid (hereinafter referred to as BON3,6) are also included in the reaction mixture.
Such hydroxynaphthalenecarboxylic acids are useful for various purposes. For example, BON6 is an important raw material for aromatic polyesters, and in particular, is an indispensable component for producing liquid crystal polymers having superior workability and high fluidity as well as resins or fibers having high elastic modulus and highly heat-resistant. BON3 and BON3,6 are also useful as raw materials for azo pigments. Similarly, alkali metal salts of hydroxynaphthalenecarboxylic acids are also useful as such. For example, the sodium salt of BON3 as such is useful as a raw material for azo pigments. The technique for separating and purifying such components from the reaction mixture is therefore quite important.
Several methods are already known for separating and purifying an intended compound from such mixtures, including, for example, those methods in which hydrochloric or sulfuric acid is added to adjust the pH in a predetermined range and thereby precipitate the intended compound or impurities, or in which the product is recrystallized using solvents (Japanese Patent Publication No. H1-216955, A (1989)), or in which the product is separated as an adduct with dioxane (Japanese Patent Publication No. H2-15046, A (1990)). Such methods, however, are not always preferred for industrial purpose because they involve a step in which alkali metal salts of hydroxynaphthalenecarboxylic acids produced are neutralized.
Disclosure of Invention
The present invention aims to separate and purify an intended compound(s) at an extremely high separation accuracy from a mixture of alkali metal salts of hydroxynaphthalenecarboxylic acids, in particular, from a mixture of alkali metal salts of 2-hydroxynaphthalene-3-carboxylic acid and 2-hydroxynaphthalene-6-carboxylic acid, without going through any completely-neutralizing step.
The present invention relates to a process for separating and purifying alkali metal salts of hydroxynapbthalenecarboxylic acids, characterized in that a mixture of alkali metal salts of hydroxynaphthalenecarboxylic acids are adsorbed to a nonionic porous synthetic adsorbent, and treated with water or water and water-soluble organic solvent(s).
The present invention further relates to the above process for separating and purifying alkali metal salts of hydroxynaphthalenecarboxylic acids, in which the nonionic porous synthetic adsorbent is an aromatic copolymer mainly composed of styrene and divinylbenzene or a methacrylic copolymer mainly composed of monomethacrylate and dimethacrylate.
The alkali metal salts of hydroxynaphthalenecarboxylic acids in the present invention are preferably those salts in the form of carboxylates.
The present invention particularly relates to the above process for separating and purifying alkali metal salts of hydroxynaphthalenecarboxylic acids, in which the mixture of alkali metal salts of hydroxynaphthalenecarboxylic acids is dissolved in water or water and water-soluble organic solvent(s), injected into the top of an column packed with a nonionic porous synthetic adsorbent, developed firstly with water or water-soluble organic solvent(s) having a high water content and then with increased proportion(s) of water-soluble organic solvent(s) in the developer.
The process of the present invention enables separation and purification of intended alkali metal salt(s) of hydroxynaphthalenecarboxylic acid(s) at high accuracy and at high yield from a mixture of alkali metal salts of hydroxynaphthalenecarboxylic acids, such as those obtained in Kolbe-Schmitt process, without neutralizing them completely, and thereby improves the productivity of such industrial processes.